1. Field of the Disclosure
The present disclosure relates to an optical waveguide and an arrayed waveguide grating, which can reduce insertion loss when light enters from a slab waveguide toward an arrayed waveguide or when the light enters from the arrayed waveguide toward the slab waveguide.
2. Discussion of the Background Art
In a DWDM (Dense Wavelength Division Multiplexing) multiplexer/demultiplexer, an M×N star coupler, a 1×N splitter, and so on, Patent Documents 1 to 6 disclose such a connection structure between a slab waveguide and an arrayed waveguide that when light enters from a slab waveguide toward an arrayed waveguide, the light does not radiate in a clad layer as a radiation mode between the arrayed waveguides adjacent to each other.
In the Patent Documents 1 to 4, a transition region where the refractive index of the waveguide gradually changes from the slab waveguide toward the arrayed waveguide is disposed. In the Patent Document 5, a slope portion is disposed between the slab waveguide and the arrayed waveguide. In the Patent Document 6, a core layer and a plurality of island-shaped regions are arranged in the slab waveguide. The refractive index of the island-shaped region is lower than the refractive index of the core layer. The island-shaped regions face a clad layer provided between the adjacent arrayed waveguides. The width of the island-shaped region in a direction substantially vertical to a light propagation direction becomes narrower from the slab waveguide toward the arrayed waveguide. Light passing through the core layer provided between the island-shaped regions adjacent to each other propagates toward the arrayed waveguide without changing the propagation direction. Light passing through the island-shaped region changes the propagation direction due to a tapered shape of the island-shaped region and propagates toward the arrayed waveguide. The tapered shape and the position of the island-shaped region are optimized, whereby the light is concentrated on the arrayed waveguide and propagates in the arrayed waveguide as a propagation mode.    Patent Document 1 U.S. Pat. No. 5,745,618    Patent Document 2 U.S. Pat. No. 7,006,729    Patent Document 3 U.S. Pat. No. 6,892,004    Patent Document 4 Japanese Patent Application Laid-Open No. 2008-293020    Patent Document 5 Japanese Patent Application Laid-Open No. 2001-159718    Patent Document 6 Japanese Patent Application Laid-Open No. 2003-14962
In the Patent Documents 1 to 4, since the transition region is disposed, the circuit size is increased. In the Patent Document 5, since the slope portion is disposed, circuit manufacturing is difficult. In the Patent Document 6, since the tapered shape and the position of the island-shaped region are required to be optimized, the circuit design is difficult.
Thus, in order to solve the above problems, the present disclosure provides an optical waveguide and an arrayed waveguide grating, which do not increase the circuit size, do not make difficult the circuit design and manufacturing, and can reduce insertion loss when light enters from a slab waveguide toward an arrayed waveguide or when the light enters from the arrayed waveguide toward the slab waveguide.